1. Field of the Invention
This invention relates to a process for the preparation of thermosetting resin powder paints. More particularly, the invention relates to a process for the preparation of thermosetting resin powder paints comprising the steps of extruding a melt of a resinous composition, cooling the composition and forming it into a sheet, and pulverizing the formed sheet.
2. Description of the Prior Art
Since powder paints include no solvent, they permit to prevent pollution or contamination not only in the paint-preparation process but also at the coating step. Accordingly, demand for powder paints has recently increased and the production of powder paints is now on the increase. Methods presently practiced industrially for the production of thermosetting resin powder paints comprise, in general, incorporating pigments, curing agents and additives into resins, blending them homogeneously by means of a premixer such as ribbon blender, dispersing the pigments and curing agents into molten resins by means of a melt-extruding machine such as an extruder, a kneader, a Banbury mixer and a heating roll, pulverizing and kneading the pigment particles by a mechanical force, withdrawing the product and cooling it, roughly grinding it, finely pulverizing the roughly ground product to obtain a particle size distribution suitable for painting, and sieving the resulting particles to obtain a powder paint product having a desired particle size distribution (see, for example, U.S. Pat. No. 3,382,295, and British Pat. Specification No. 1,043,998 U.S. Pat. No. 3,725,340 describes the preparation of cross-linkable lacquer resins by continuously reacting melts of lacquers resins containing hydroxyl groups with alkoxymethyl isocyanates in screw extruders. The liquid alkoxymethyl isocyanate is added to the melt of the resin containing hydroxyl groups and the reaction is completed in a few minutes).
As another special method for the preparation of powder paints, there can be mentioned a method for preparing powder resins which comprises expanding a resin to form a foamed structure and subjecting the foamed structure to comminution to produce particles (see British patent Specification No. 1,079,262), and a method for preparing powder paints which comprises concentrating a solution of a resin under reduced pressure by means of a self-cleaning screw evaporator, rapidly cooling the resulting viscous melt by cold air or in water or other liquid, and pulverizing the resulting solid to produce a powdery composition (see British patent Specification No. 1,353,351).
In general, methods for the production of powder paints presently worked commercially comprise steps of material compounding, pre-mixing, melt kneading and dispersion, rough grinding, comminution, (sieving) and product recovery. These steps will now be described in detail.
A pre-mixture of a pigment, a resinous component and additives is continuously kneaded by a melt-kneading machine (extruder) and extruded from a rod die of the extruder, and the extrudate is elongated into a sheet having a thickness of about 2 mm by means of a pair of rolls. Then, the sheet is cooled by cold air while it is being transported on a conveyor roll, or it is naturally cooled while it is being moved on a cooling roll. Then, the cooled sheet is roughly ground into ships having a size of 10 mm .times. 30 mm and the ships are comminuted. With increase of the extrusion rate (Kg/hour) in the extruder, the equipment for the step of cooling the extrudate and forming it into a sheet must inevitably be made larger to cope with the increased extrusion rate and depending on the temperature of the extrudate and properties of the resin, and operations for this step become complicated and troublesome.
Extrusion rates of extruders presently used for the production of powder paints are 80 to 150 Kg/hour at the most, and temperatures of molten extruded resins are about 100.degree. to about 130.degree. C. The steps of cooling such extrudates and forming them into sheets are generally performed by expanding a molten resin extrudate by using a pair of rolls having a diameter of about 200 mm and a length of about 600 mm which are rotated at a rate of 7 to 10 rpm, to thereby form the extrudate into a sheet having a thickness of about 2 mm and a width of about 200 mm, and naturally cooling the sheet while transporting it on a conveyor (having a length of about 5 to about 10 mm) moved at a speed of 6 to 12 m per minute or feeding cold air into the sheet by a fan or cooler to effect forced cooling according to need.
FIG. 1 shows diagrammatically in section an example of an apparatus for practising the above conventional method, in which reference numerals 6, 7, 8, 9 and 10 denote a material inlet, an extruder, a pair of cooling rolls, a conveyer and a granulator, respectively.
As another conventional method, there can be mentioned a method comprising sprinkling cold water at the outlet of expanding rolls to promote cooling of a sheet of a molten expanded resin, a method comprising cutting the as-extruded resin in water or cold air by the strand process and a method comprising cutting the extruded sheet after water cooling.
However, if the extrusion rate and the manufacturing rate are to be increased so as to satisfy the demand for continuous mass-production, and for example, the extrusion rate of the extruder is to be increased to 300 to 500 Kg/hour, then in the foregoing conventional methods a large quantity of cold air must be fed to rapidly and uniformly cool a large quantity of a high-temperature, melt-extruded resin, and the length and width of the cooling conveyor must be made larger. Accordingly, the size of the production plant must also be increased. Therefore, according to the conventional method, it is industrially difficult or impossible to increase the extrusion rate.
The method comprising sprinkling water on melt-extruded resins to cool them is defective in that a part of curing agents and other additives incorporated in thermosetting powder paints are dissolved in the water, the water content of the powder paint is increased, and the quality of the product is degraded drastically.
Further, in the case of powder paints of the low temperature-baking type, since a long time is required for complete cooling according to the above-mentioned cooling method, the curing reaction cannot be avoided during the cooling step.